Density Functional Theory Study On Photocatalytic Activities Of Doped And Heterostructured Wide Band Gap Semiconductors

Wide band gap semiconductor photocatalysts have drawn much attention because of their potential application in photocatalytic water splitting for hydrogen and photocatalytic decomposition of harmful substances.The wide band gaps of wide band gap semiconductor photocatalysts limit the utilization of ultraviolet light accounting for only 4% of the solar energy,high recombination rate of electron and hole,and conduction or valence band edge position unfavorable for oxidation and reduction processes for decomposition of water or pollutants have limited photocatalytic efficiency of many photocatalysts,so finding ideal visible light photocatalyst is full of challanges.First principle calculations based on hybrid density functional theory have been adopted to investigate the effect of foreign doping and construction of heterojunction on the photocatalytic activities of wide band gap semiconductors of KNbO₃,NaNbO₃,BiOX?X = F,Cl,Br,I?,SrTiO₃,NaTaO₃ and g-ZnO,so as to improve the visible light absorption and photocurrent density.The effect of different dopings and construction of heterojunction have been adopted to reveal the mechanism of improvement photocatalytic activities induced by doping and building heterostructure,so as to design an ideal photocatalyst.The main researches are listed as follow:1.The strategies of charge compenstated codoping have been adopted to regulate the photocatalytic activities of?K,Na?NbO₃.The results suggest that Cr–N codopings can effectively reduce the band gaps,but the the introduction of unoccupied impurity states acting as the recombination of electron-hole pairs will suppress the photocatalytic activity.Mo–N or N–F codopings could reduce the band gaps of?K,Na?NbO₃with the band edge positions straddling the water redox level without the introduction of impurity states,which will be beneficial for visible photocatalysis for water splitting.2.Double-hole-mediated codopings have been adopted to tune the photocatalytic activity of KNbO₃.The results suggest that the band gaps of Ti–N,Zr–P,Zr–N,Sc–S and Y–S codoped systems are obviously reduced,but the unoccupied impurity states between the valence and conduction bands serving as electron-hole centers will affect the photocatalytic efficiency.The codopings of V–C,Ti–P,N–N,P–P,N–P,and C–S could reduce the effective band gap without the introduction of unoccupied impurity states.The intermediate states of P-P,N-P and C-S codoped systems are close to or higher than the water reduction potential,so it is thermodynamically unfavorable for water oxidation and reduction progresses.With appropriate band gaps and band edge positions,V–C codoped KNbO₃ are ideal photocatalysts for water splitting.Ti–P and N–P codoped KNbO₃ are suitable for Z-scheme photocatalysis.3.The photocatalytic activities of BiOX/BiOY?X = F,Cl,Br,I?heterostructures have been investigated.The results suggest that the band gaps of BiOX/BiOY systems are between band gap values for BiOX and BiOY systems.The calculated band gaps for BiOF/BiOCl,BiOF/BiOBr,BiOF/BiOI,BiOCl/BiOBr,BiOCl/BiOI,and BiOBr/BiOI are respectively 3.86,3.41,2.74,2.99,2.30,and 2.23 eV.The sequence of the maximum absorption wavelength is BiOF < BiOF/BiOCl < BiOCl < BiOF/BiOBr < BiOCl/BiOBr < BiOBr <BiOF/BiOI < BiOCl/BiOI < BiOBr/BiOI < BiOI.Due to the conduction band maximum is lower than the hydrogen reduction potential,all the BiOX and BiOX/BiOY systems are thermodynamically unfavorable for hydrogen production.Meanwhile,the suitable band gaps and band edge positions make BiOF/BiOI,BiOCl/BiOBr,BiOCl/BiOI,and BiOBr/BiOI superlattices possible visible light photocatalysts for degradation of organic pollutants.4.The photocatalytic activities of Srm/Nan??SrTiO₃?m/?NaTaO₃?n?with with different layers are investigated.The results show that the band gaps of all the SrTiO₃/NaTaO₃ heterostructures are smaller than those of SrTiO₃ and NaTaO₃.For SrmNan?m + n = 10?,the band gap decreases in the order of Sr1Na9,Sr2Na8,Sr3Na7,Sr4Na6,Sr5Na5,whereas the band gap increases in the sequence of Sr5Na5,Sr6Na4,Sr7Na3,Sr8Na2,Sr9Na1.SrmNam?m = 1,2,3,4,6,7?heterostructures are built to find the smallest band gap of SrTiO₃/NaTaO₃ heterostructure,and the Sr6Na6 heterostructure has the smallest band gap of 2.58 eV in our considered structures.Sr6Na6 has exhibited a obvious shift of optical absorption towards the visible light region as compared to those of SrTiO₃ and NaTaO₃,and the band edge positions are suitable for photocatalytic water splitting.The construction of SrTiO₃/NaTaO₃ heterostructures is an effective strategy to reduce the band gaps of SrTiO₃ and NaTiO₃to enhance the photocatalytic activities.5.The photocatalytic activities of ZnO/MoS2 and ZnO/MoSe2 composites are explored.The calculated results suggest that the ZnO/MoS2?ZnO/MoSe2?heterostructure is easy to be synthesized because of the negative interface adhesion energies and the weak vdW interactions between the ZnO and MoS2?MoSe2?monolayers.The charge density difference and bader charge analysis indicate that electrons transfer from ZnO side to MoS2?MoSe2?side,and this character will promote the efficient separation and transportation of photo-generated carriers and thus is favorable for improving the photocatalytic efficiency.The ZnO/MoS2 and ZnO/MoSe2 composites could absorb enough visible light.The band edge positions of ZnO/MoS2 is unfavorable for the water reduction process,while the band edge positions of ZnO/MoSe2 is unfavorable for the water oxidation process.The ZnO/MoS2 heterostructure with the strain of-2% and ZnO/MoSe2 heterostructure with the strain of +2% have proper bandgaps with the band edge positions straddling the water redox level,so it is thermodynamically favorable for water photooxidation and photoreduction processes.